Sorry for taking a while to get back to you.
I see no reason why you can't have one male Catholic Godparent and one female Catholic Godparent for both twins as long as they are faithful Catholic witnesses to the newborns.
That said, we receive more than a few e-mails from parents who are disappointed with their choice of Godparents so, for the good of the children, it may be more prudent to have more Godparents than fewer: 4 Godparents rather than 2.
This is what Canon law states:
Chapter IV. — Sponsors
Can. 872 Insofar as possible, a person to be baptized is to be given a sponsor who assists an adult in Christian initiation or together with the parents presents an infant for baptism. A sponsor also helps the baptized person to lead a Christian life in keeping with baptism and to fulfill faithfully the obligations inherent in it.
Can. 873 There is to be only one male sponsor or one female sponsor or one of each.
§1. To be permitted to take on the function of sponsor a person must:
- be designated by the one to be baptized, by the parents or the person who takes their place, or in their absence by the pastor or minister and have the aptitude and intention of fulfilling this function;
- have completed the sixteenth year of age, unless the diocesan bishop has established another age, or the pastor or minister has granted an exception for a just cause;
- be a Catholic who has been confirmed and has already received the most holy sacrament of the Eucharist and who leads a life of faith in keeping with the function to be taken on;
- not be bound by any canonical penalty legitimately imposed or declared;
- not be the father or mother of the one to be baptized.
§ 2. A baptized person who belongs to a non-Catholic ecclesial community is not to participate except together with a Catholic sponsor and then only as a witness of the baptism.
I hope this help,